IPA: /səˈpoʊzəl/
KK: /səˈpoʊzəl/
The act of thinking something is true or possible without having proof; an idea or belief that is not certain.
Her supposal that he would arrive late turned out to be correct.
Supposal is formed from "supponere" (meaning to place under) and the suffix "-al" (meaning pertaining to). The word refers to something that is placed under consideration or assumed.
Think of something being 'placed under' ('supponere') for consideration — that's why supposal means an assumption or something assumed.